The goal is to understand how neurons in pattern-generating circuits function and contribute to orderly movements. Local interneurons are the most abundant neurons in brains, but because they are small and difficult to record, little is known about their physiological characteristics. A small set of axonless, nonspiking local interneurons has been discovered in the crayfish that is part of the neural circuits that generate rhythmic swimming movements. These interneurons are identifiable, and large enough to permit experiments that explore their physiological properties and synaptic interactions. We propose three projects that will describe how these nonspiking local interneurons contribute to the swimmeret rhythm. To test the role of these interneurons in generating the swimmeret movements, individual interneurons will be perturbed or ablated while the nervous system is generating the swimmeret rhythm. The membranes of interneurons will be clamped to different potentials with a single-electrode voltage clamp to control their release of transmitter. Individual interneurons will be ablated by photoinactivation. Comparisons of the motor patterns generated before and during the voltage clamp or before and after ablation will reveal the contribution of these interneurons to the motor pattern in their own circuit and in the whole animal. To test the hypothesis that nonspiking local interneurons are premotor, and physiologically remote from sensory input, pairs of identified motorneurons, sensory neurons and interneurons will be injected with Lucifer yellow and HRP and then studied to describe and count their points of contact. Regions of apparent contact will be thin-sectioned for EM to locate any synapses. To analyze how these interneurons integrate synaptic currents, we will describe their passive electrical structure in a detailed compartmental model by combining measurements of their pulse-responses and step-responses with careful measurements of their anatomical structure. The predictions of this model will be tested with a single-electrode voltage clamp to measure synaptic currents and synaptic potentials from known sources. This analysis will test the hypothesis that "subunits" of synaptic integration exist in these dendritic structures.